Thin film microwave iris

ABSTRACT

A thin film microwave iris including an aperture, inside a microwave waveguide transverse to the flow of microwave energy in the waveguide with the thin film electrically connected to the walls of the waveguide. Due to the flow of energy through the film as well as through the aperture, the characteristics of thin film irises are different from the characteristics of conventional irises.

United States Patent [151 3,676,809 Paine et al. [45] July 11, 1972 s41 THIN FILM MICROWAVE IRIS OTHER PUBLICATIONS [72] T. O. Paine, Administrator of the National Ramey et al., Microwave Properties of Thin Films with of Technology. April 1960, pp. 163- 166 Ragen, G. L., Microwave Transmission CZircuits, McGraw [22] Filed: May 28, 1970 Hill l948,pP- 220-222 [211 Appl 41345 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. l-l. Punter 52 us. Cl. ..333/98 R, 333/24 R, 333/98 P Attorney-John s Howard Osborn and William [51] ..l-l0lp 1/00, l-l0lp 5/02 K1118 [58] Field ol Search ..333/98 P, 98 ABSTRACT [56] References Cited A thin film microwave iris including an aperture, inside a microwave waveguide transverse to the flow of microwave UNITED STATES PATENTS energy in the waveguide with the thin film electrically connected to the walls of the waveguide. Due to the flow of ener- Z lolgon 2 8y through the film as W as thrpugh the aperture, the 2834949 5/1958 333/98 P characteristics ofthin film lrises are different from the characterisfics ofconventional irises. 3,101,460 8/1963 Walker et a1. ..333/98 P 3,448,413 6/1969 Preist et a1. ..333/98 1 Claim, 1 Drawing Figure Inventors:

Y Aeronautics and Space Administration with respect to an invention of; Robert L. Ramey; l-lugh S. Landes, both of Charlottesville; Eugene A. Menus, Blacksburg, all

Apertures," MTT- 18, No. 4, 1970, pp. 196-204 Southworth, G. C., Principles & Applications of Waveguide Transmission, D. Van Nostrand Co., 1950, pp. 244- 255 l-larrowell, R. V., Elliptic Waveguide Windows, Electronic INVENTORS ROBERT L. RAMEY HUGH S. LANDES EUGENE A. MANUS THIN FILM MICROWAVE IRIS ORIGIN OF THE INVENTION BACKGROUND OF THE INVENTION The invention relates generally to irises in microwave waveguides and more specifically concerns a thin film iris including an aperture therein in microwave waveguides.

Thin film as used in this specification and claims include all film thicknesses for which a detectable power transmission through the film can be measured provided that the thickness of the film is negligible when compared to the waveguide wavelength of the microwave signal. To be more specific, in the case of all metal and semimetal films, thin film includes all film thickness of less than 1,000 angstrom units even though the thickness can be of several thousand angstrom units and still meet the definition. Low conductivity semiconductors, on the other hand, could require thickness on the order of wavelength and therefore can not be considered as films." High conductivity semiconductors may be on the order of tens of thousands of angstroms in thickness and satisfactorily meet the requirement of being a thin film.

The recent introduction of thin conducting films into the fabrication of microwave devices opens a new area of research and development. Because thin conducting films have a finite conductivity (as contrasted to the almost perfect conductivity of most bulk metals) they can support an electric field. Thus, the conventional boundary condition that the tangential component of electric strength is zero at a conducting surface does not apply to these films with their finite conductivity.

In a conventional iris the flow of energy is only through the aperture whereas in a thin film iris there is flow of energy through the thin film as well as through the aperture. Also, there is a finite electric field over the surface of the thin film iris. These properties of the thin film iris cause it to have characteristics different from the characteristics of a conventional iris. The power transmission through the thin film iris is dependent on the thin film conductivity times the film thickness, and on the shape and size of the aperture in the film. A thin film iris can be designed to present either an inductive or a capacitive reactance. A rectangular aperture, which is normally inductive in conventional irises, may be capacitive under predictable conditions in the thin film iris. Elliptical apertures in thin films do not display capacitive reactances provided the minor axes of the ellipses are kept relatively small, and are oriented less than 45 from a parallel to the narrow wall of the waveguide.

SUMMARY OF THE INVENTION The invention includes a thin film in a waveguide with the edges of the thin film connected electrically to the walls of the waveguide. A small aperture is cut in the film to control the power transmission through the film. The shape of the aperture can be either circular, rectangular or elliptical and be made either capacitive, inductive or resistive by merely changing the size of the aperture.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE in the drawing is a schematic drawing of the preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Turning now to the embodiment of the invention selected for illustration in the drawing the number 11 designates a microwave waveguide. A thin film 12 is deposited on a thin mica substrate and located in waveguide 11 transverse to the flow of energy through the waveguide. Mica substrates several thousandths of an II'ICI'I in thlckness are transparent and do not the minor axis then it becomes capacitive. Hence by merely I changing the major axis relative to the minor axis of the elliptical aperture of a thin film his the iris can be made inductive, resistive or capacitive.

Measurements have also been made on thin film irises with rectangular apertures. These measurements show that an iris is capacitive provided the minor axis is kept relatively small. Then as the minor axis of the aperture is made larger the iris displays an inductive reactance. Hence the rectangular aperture too can be changed in size to cause the thin film iris to be capacitive, inductive or resistive. Similar measurements have been made on a thick foil iris of identical dimensions with the rectangular aperture being varied in the same way that it was varied in the thin film measurements. These measurements show that a thick foil iris exhibits a positive or an inductive reactance at all times and never becomes capacitive. Consequently, the thin film iris with a rectangular aperture has the advantage over a similar foil iris with a rectangular aperture in that it can be made to exhibit either a capacitive, inductive or resistive impedance to the flow of microwave energy through the waveguide while the foil iris remains inductive.

The advantage of this invention is that it is adaptable to more applications than previous microwave irises.

What is claimed is:

1. A thin film microwave iris for a waveguide comprising: a thin film of conducting material inside said waveguide trans verse to the flow of microwave energy inside said waveguide; said film being of such thickness as to both transmit and obstruct a substantial portion of the incident microwave energy; and an aperture in said thin film whereby the impedance of said thin film can be made inductive, capacitive or resistive by changing the size of said aperture and whereby the impedance properties of said iris is substantially difi'erent than that of a geometrically similar iris in a thick conductive diaphragm. 

1. A thin film microwave iris for a waveguide comprising: a thin film of conducting material inside said waveguide transverse to the flow of microwave energy inside said waveguide; said film being of such thickness as to both transmit and obstruct a substantial portion of the incident microwave energy; and an aperture in said thin film whereby the impedance of said thin film can be made inductive, capacitive or resistive by changing thE size of said aperture and whereby the impedance properties of said iris is substantially different than that of a geometrically similar iris in a thick conductive diaphragm. 